String test, with an OR connector

Question

In this post, a solution is given to the problem of defining a command which tests equality of two strings, sufficiently robust (in comparison to \ifthenelse) to admit complicated arguments (e.g. containing commands or environments). But now, i would like to adapt it to perform a test on 1 argument in the form "IF arg= a OR arg=b THEN ... ELSE ...", where "a" and "b" or fixed. My attempt is :

\makeatletter
\newcommand{\testOR}[1]{%
  \ifnum\pdfstrcmp{\unexpanded{#1}}{a}=\z@
    \expandafter\@secondoftwo
  \else
    \ifnum\pdfstrcmp{\unexpanded{#1}}{b}=\z@
            \expandafter\@secondoftwo%
    \else%
            \expandafter\@firstoftwo%
    \fi%
  \fi
}
\makeatother
\begin{document}

\testIfNotTrivial{a}{true}{false} - 
\testIfNotTrivial{b}{true}{false} - 
\testIfNotTrivial{coucou}{true}{false}

\end{document}

But it does not have the expected (from me) behaviour : false - False - true

Answer 1

Without a MWE this is untested but in your second branch the\expandafter only gets rid of one \fi you still have the outer \fi left so looks like you would want

\ifnum\pdfstrcmp{\unexpanded{#1}}{b}=\z@
        \expandafter\expandafter\expandafter\@secondoftwo
\else
        \expandafter\expandafter\expandafter\@firstoftwo
\fi

(no need for % after a command name)

Answer 2

You really should give LaTeX3 a try, if you have many tests like this:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\testOR}{m}
  {
   \bool_if:nTF
     {
      \str_if_eq_p:nn { #1 } { a } || \str_if_eq_p:nn { #1 } { b }
     }
  }
\ExplSyntaxOff
\begin{document}
\testOR{a}{true}{false} --
\testOR{b}{true}{false} --
\testOR{coucou}{true}{false} --
\testOR{\textbf{coucou}}{true}{false}
\end{document}

This gives

true – true – false – false